Porous separation membranes suitable as ultrafiltration membranes and supports for reverse osmosis membrane, nanofiltration, and pervaporation are generally formed by dissolving the polymer in a solvent, casting the polymer solution on a support as a thin film, and coagulating the polymer by immersion of the support and wet thin film in a coagulation bath. Attempts to prepare solvent-resistant microporous membrane from solvent-inert polyimides and polyketones have been carried out over the years. The various methods for producing a porous membrane using a phase inversion process are known in the art. See R. Kesting, Phase-Inversion Membranes in Synthetic Polymeric Membranes-A Structural Perspective, 237-286, John Wiley & Sons (1985).
Aromatic polyether ketones and polyimide can be suitable materials for solvent-resistant membranes due to their intrinsic solvent-inertness. Various approaches utilizing aromatic polyether ketones and polyimides are known. For example, German Pat. No. 3,321,860, European Pat. No. 182506, and U.S. Pat. Nos. 6,017,455 and 5,997,741 describe the use of concentrated sulfuric acid to dissolve polyketones. U.S. Pat. No. 4,992,485 describes the preparation of filtration membranes by dissolving polyarylene ether ether ketone (“PEEK”) in an organosulfonic acid that does not sulfonate PEEK. U.S. Pat. Nos. 4,755,540 and 5,227,101 describe the melt processes of preparing porous separation membranes in which high temperature is used. U.S. Pat. No. 4,957,817 discloses the processes of preparing porous membranes using an organic polar solvent having a boiling point in a range from 141° C. to 380° C., and casting the PEEK solution at high temperature. U.S. Pat. Nos. 3,925,211 and 4,071,590, and Strathmann (26 Desalination 85 (1978)) describe the process of preparing an asymmetric membrane from a soluble film-forming prepolymer and converting it into a final membrane product that is insoluble in solvents. These membranes can be brittle and have poor-mechanical properties. Hence, there remains a need for improved solvent-resistant microporous membranes from solvent-inert polyimides and polyketones.